A power amplifier (PA) is used in a wireless communication device such as a cellular radiotelephone to amplify radio frequency (RF) signals so that the device can communicate with a fixed site transceiver. Considerable power in a wireless communication device is dissipated in the power amplifier. For example, in a portable cellular radiotelephone, a significant percent of the power dissipation is in the power amplifier. Efficiencies of a power amplifier significantly depend upon the source and load variations the power amplifier experiences over the operating frequency of a cellular radio telephone. One problem associated with designing a high-efficiency power amplifier is adequately compensating for these source and load variations.
Wireless communication devices typically transmit radio frequency signals at a plurality of power levels. For example, cellular radiotelephones require seven 4 dB steps in output power of the radio transmitter. However, the efficiency of the power amplifier significantly varies over the output power range. Because current drain efficiency of the power amplifier is most affected at a higher output power, the power amplifier is designed to maximize efficiency at higher output power levels. One technique to improve power efficiency requires switching the quiescent current of the power amplifier in response to a power amplifier output step change. At the lowest power step, the power amplifier is normally in class A mode of operation. By changing the bias conditions of the power amplifier at the lower steps, the power amplifier could be kept in class AB mode with a corresponding improvement in efficiency.
Operational efficiency is also particularly dependent on the load impedance variation caused by impedance changes of the duplex filter over a wide bandwidth or environmental factors such as the placement of a user's hand or body near the antenna. The power amplifier is generally designed with the saturated output power higher than the typical operating output power to accommodate power fluctuations caused by load impedance variations. To compensate for the reduced efficiency of the power amplifier designed at such a higher output power, the load at the output of the power amplifier can be varied by switching a diode in or out at the output. For example, a dual mode power amplifier operates in either the linear mode or the saturation mode based upon load switching. This load switching accommodates operation of the device on two different cellular radiotelephone systems. However, such discrete switching of the load at the output provides some improvement in effieciency, but does not maximize efficiency.
Improving the power amplifier efficiency is essential to increasing the operating time for a given battery of the wireless communication device. Accordingly, there is a need for a method and apparatus for amplifying a radio frequency signal with greater efficiency over source and load variations. There is also a need for operating a power amplifier efficiently over a wide range of operating voltages.